Secondary batteries are becoming widely used as power supplies for vehicles and household appliances, and not only as power supplies for portable devices such as mobile phones, digital cameras and laptop computers, and among others, lithium ion secondary batteries, which have a high-energy density and are lightweight, are energy storage devices that are indispensable in daily life.
Secondary batteries are generally classified into a rolled type and a stacked type. A battery electrode assembly of a rolled type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet laid on each other via a separator are rolled a plurality of turns. A battery electrode assembly of a stacked type secondary battery has a structure in which positive electrode sheets and negative electrode sheets are alternately stacked with separators interposed therebetween. The positive electrode sheets and the negative electrode sheets each include a current collector including a coated portion to which active material (which may be a compound agent containing, e.g., a binder and a conductive material) has been applied and an uncoated portion to which active material has not been applied in order to allow an electrode terminal to be connected thereto.
In either a rolled type secondary battery or a stacked type secondary battery, a battery electrode assembly is enclosed inside an outer container in such a manner that: one end of a positive electrode terminal is electrically connected to an uncoated portion of a positive electrode sheet and another end of the positive electrode terminal extends to the outside of the outer container (outer case); and one end of a negative electrode terminal is electrically connected to an uncoated portion of a negative electrode sheet and another end of the negative electrode terminal extends to the outside of the outer container. Inside the outer container, in addition to the battery electrode assembly, electrolyte is enclosed. Capacities of secondary batteries have been increasing year by year, and along with this increase, heat that would be generated if a shortcut occurs also increases, causing an increase in risk, and thus, measures to ensure battery safety are becoming increasingly important.
As an example of a safety countermeasure, a technique in which insulating material is formed on a boundary part between a coated portion and an uncoated portion in order to prevent a shortcut between a positive electrode and a negative electrode is known (Patent Document 1).